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California’s water supplies are at risk as climate change increases natural dry spells, especially in California’s Sierra Nevada.
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California’s water supplies are at risk as climate change increases natural dry spells, especially in California’s Sierra Nevada.

A large, manmade canal flows through low hills.


California is bracing for a third consecutive year of drought. Officials are increasing water restrictions to levels never before seen in the water year. Most of the state’s water reservoirs are well below averageMany of them at less than a third their capacity. The outlook for rain and snow this winter, when most of the state’s yearly precipitation arrives, isn’t promising.

Particularly worrying is the outlook on the Sierra Nevada, a long mountain chain that runs through the eastern half of California. California’s cities and its farms – which grow over a third of the nation’s vegetables and two-thirds of its fruit and nuts – rely on runoff from the mountains’ snowpack for water.

As an engineer, I have studied California’s water and climate for over 30 years. A closer look at California’s water resources shows the challenge ahead and how climate change is putting the state’s water supply and agriculture at greater risk.

California’s water sources

California experiences an average of about 2 feet of precipitation annually, which is approximately two-thirds of what the global average is. This gives it a semi-arid climate.

The majority of California’s rain and snow falls in the mountains, primarily in winter and spring. However, water is essential for agriculture and coastal cities to survive the dry summers. California has developed over the past century to provide water for dry Southern California and flood control in the north. a statewide systemThere are many tunnels, reservoirs and canals that bring water up the mountains. The State Water Project is the largest project. It delivers water from the northern Sierra with higher precipitation to the southern part of the state.

A large, manmade canal flows through low hills.
A section of the California Aqueduct in the State Water Project.
Ken James/California Department of Water Resources

To track where the water goes, it’s useful to look at the volume in acre-feet. California covers approximately 100 million acres. At 2 feet per annum, California’s annual precipitation is approximately 200 million acre-feet.

There are only 200 of these, and the average is about 80 million acre-feet. Most of the water is returned to the atmosphere through evapotranspirationTrees and plants in the Sierra Nevada, or North Coast forests. About half of the 80,000,000 acre-foots that is runoff remains in the water environment, such rivers flowing to oceans. This leaves 41 million acre-feet available for downstream use. About 80% goes to agriculture, and 20% to urban uses.

In wet years, water may be available in excess of 80 million acre-feet. However, in dry years it can be even lower.

In 2020, for example, California’s precipitation was less than two-thirds of average, and the State Water Project delivered only 5 % of the contracted amounts. The state’s other main aqueduct systems that move water around the state also severely reduced their supplies.

The 2021 water-year, which ended Sept. 30, had been one of the driest ever recorded for the Sierra Nevada. Precipitation was approximately 44% of average. There was little precipitation in December 2021, and the state is still in extreme droughtThe State Water Project lowered its initial allocations to water agencies to 0% by 2022. Small amounts are still available for safety and medical needs.

Although conditions could improve if there are more storms in the next three-months, the official outlook of the National Oceanic and Atmospheric Administration is to remain stable. below-normal precipitation being more likely than above normal.

Comparison of state maps with water uses in wet and dry years

California State Water Plan 2018

Drought and a warming environment

Multiyear dry periods, when annual precipitation is below average, are a feature of California’s climate, but rising global temperatures are also having an impact.

In the last 1,100 years, at least one dry period has occurred every century for four years or more. There have been two in the past 35 years – 1987-92 and 2012-15. These dry periods are exacerbated by a warmer climate. Dryer soil and dry air stress both natural vegetation as well as crops.

Rising global temperatures impact runoff from Sierra Nevada, which provides drinking water. over 60% of California’s developed water supply.

More than 80% of the runoff from the central and southern Sierra Nevada is derived from the snow zone. Rainfall contributes more to the northern Sierra, which is wetter and lower-elevation. one-third of the annual runoff.

The average snowline, which is the elevation at which snowfall is most intense, ranges from approximately 5,000 feet in the north to about 7,000 feet in south. On average, each 1.8 degrees Fahrenheit (1 Celsius) of warmingCould push the snowline an additional 500 feet higher, reducing snow total.

Flood control will take more capacity from existing dams, further reducing their capacity to store seasonal water-supply.

A dry ring around the reservoir shows how low its water level is.
A section of Shasta Lake, California’s largest reservoir, on Oct. 28, 2021.
Andrew Innerarity/California Department of Water Resources

A wealth of researchThe Sierra Nevada could see, according to research. low- to no-snow winters for years at a time by the late 2040s if greenhouse gases emissions don’t decline, with conditions worseningYou can go further than that.

Also, warming will be increase water demand from forests as growing seasons lengthenBoth. drought stress leading to tree mortalityIncreasing risk of high-severity wildfires.

Sustainability in a warming environment

Water storage is central to California’s water security.

Both farms and communities can be rehabilitated. pump more groundwaterWhen supplies are low, the state pumps out more water than it has in years. Parts of the state are dependent on water from Colorado River. Its dams provide several years of water storage. But the basin is not. lacks the runoff to fill the dams.

Public opposition has made new dams difficult to construct. It is therefore crucial that groundwater be used for storage both seasonalally and multiyears.

Aerial view of a recharge ponds
Groundwater banking is a way to recharge groundwater during dry periods. This is essential for weathering long-term droughts. These shallow ponds allow water from the ground to soak into underground wells.
Dale Kolke/California Department of Water Resources

The state’s Sustainable Groundwater Management ActLocal agencies are required to develop sustainability plans. This provides some hope that groundwater pumping, replenishment and cropland can be brought into balance. The managed aquifer recharge south from the Sacramento-San Joaquin Delta has been steadily expanding. There is still much to be done.

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If the state doesn’t do more, including tactics such as applying desalination technology to make saltwater usable, urban areas can expect the 25% cuts in water use put in place during the 2012-15 drought to be more common and potentially even deeper.

California’s water resources can provide for a healthy environment, robust economy and sustainable agricultural use. Achieving this will require upgrading both natural infrastructure – headwaters forests, floodplains and groundwater recharge in agricultural areas – and built infrastructure, such as canals, spillways and levees. The information is availableOfficials now need to follow through.


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